Mighty River Power says a new geothermal power station at Kawerau is now operational a month ahead of schedule.
The $300 million station will add another 100MW to New Zealand's generation capacity, which is under pressure as the South Island hydro lakes continue to fall.
Mighty River says Kawerau will contribute significantly to security of supply over winter.
The power plant meets about one-third of residential and industrial demand in the eastern Bay of Plenty.
Mighty River says it the largest single geothermal development in New Zealand in more than 20 years.